1. Field of the Invention
The present invention relates to data storage systems, and more particularly to data storage systems that store data in tracks on a storage medium. Still more particularly, the present invention relates to a method and system for writing concentric data tracks in a data storage system.
2. Description of the Prior Art
Data storage systems record and reproduce data stored on a recording medium. Conventional systems typically include one or more storage disks in the storage system. The recording medium generally takes the form of a circular storage disk having a plurality of concentric data tracks formed thereon. Magnetic and optical disks are two examples of storage disks that are used in data storage systems.
The positioning of one or more recording heads relative to the data tracks is generally accomplished by incorporating a closed-loop, electro-mechanical servo system. The implementation of the servo system may include a dedicated servo surface that is associated with one of the plurality of heads in the data storage system. Alternatively, short bursts of servo data, referred to as a servo burst field, may be written amid the contents of the user data tracks. One technique used to form the servo data on a disk is to use a servo track writer. A servo track writer writes the servo data and a control system monitors the servo surface or the servo burst field data to maintain the position of the heads relative to the concentric tracks on the disk(s). Unfortunately, as track density increases, conventional servo track writers are unable to write servo data with the necessary accuracy.
Another limitation to conventional storage systems relates to the manufacturing tolerances of the system. When the storage system is assembled, the disks are typically placed on a shaft that fits into a hole at the center of each disk. The disks are then clamped into place in order to secure each disk in a fixed plane, one above the other. Each disk will then rotate around the shaft at its corresponding center.
Unfortunately, the positioning of the disks relative to the shaft is influenced by various tolerances, including shaft and clamp tolerances. FIG. 1 depicts a data track written on a patterned storage disk. A recording disk 100 is rotated by a spindle motor mounted at the center 102 of the recording disk 100. The solid line 104 represents the ideal servo track. The dashed line 106 represents the track center after the servo track writing process. Due to various disturbances that occur during the servo track write process, as well as media imperfections, track 106 is not a concentric circle on the disk. Instead, track 106 has been written eccentric in relation to the center of the disk 100 (or to the center of balance of the disk).
Additionally, the tracks are not necessarily lined up from one disk surface to another disk surface. This is known as radial misalignment. Radial misalignment and eccentricity mean the servo system has to work harder to follow the track and to find the same track on another disk.
Furthermore, the distortion of the disk due to clamping, repeatable motor runout, and other factors may cause additional higher harmonic disturbances. FIG. 2 illustrates a portion of a data track written on a recording disk. Data track 200 is comprised of an ideal track center 202 and an actual track center 204 as written by the writer. The width of the data track is approximately five micro-inches. Consequently, when the recording head attempts to follow the actual track center, additional higher harmonic disturbances are created. The variations in the actual track center occur over such as small distance (the track width) and can change directions so quickly, that the head has trouble following the actual track center. And in doing so, it creates additional higher harmonic disturbances in the storage system.
The present invention overcome the limitation of the prior art by providing a method and system for writing concentric data tracks in a data storage system. The method provides for self servo writing by first calibrating a ruler on a disk surface. Correction factors to correct for the eccentricity are determined. The final servo patterns are then written and the correction factors are modified to account for any variation in the repectable disturbances and for errors caused by any non-repeatable disturbances.
To begin the process a ruler is calibrated. A servo system is then activated and the application of the correction factors determined in the previous step. The servo system is now running on corrected circular tracks because the correction factors are being applied. Next, an A burst is written and the correction factors are adapted to account for repeatable disturbances that vary at various points on the disk surface. The recording head is then moved forward by xr/4, and the C burst is written and the correction factors are adapted. The recording head is moved forward by xr/4, and the B burst is written and the correction factors are adapted. Again, the recording head is moved forward by xr/4, and the D burst is written and the correction factors are adapted.
After the final servo patterns are then written and the correction factors are modified for any variations in the repeatable disturbances, the drive level ZAP correction factors are determined when the magnitude of the signal read from the A and B bursts is equal. Typically, moving the head back by xr/2 locates a position where A=B. Next, the drive level ZAP correction factors for C=D are determined (move head forward by xr/4). The ZAP correction factors are then stored on the disk, typically in the dedicated fields in the servo sectors. During normal operation of the storage system, the ZAP correction factors are read as part of the drive level servo sectors and subtracted from the measured position. Once the drive level ZAP correction factors are determined and stored on the disk, the head is moved to the next track (move forward xr/2), the process repeats until all of the tracks have been completed.